Various cross-stack monitors are used for industrial pollution monitoring or combustion control—for example opacity monitors measuring dust or CO monitors measuring CO.
These are optical instruments. A beam of light is transmitted across the stack and analysed. Sometimes the source is at one side of the stack and the receiver (analyser) at the other side. Sometimes source and receiver are put together on one side of the stack and a reflector is placed on the other side.
It is important to protect the instrument from stack gases—which may be hot, dirty and corrosive. This cannot be achieved simply by use of windows, since these get dirty and this influences calibration. It is typically achieved by windows in combination with air-purged sight tubes.
In a typical arrangement sight tubes are welded to the stack and clean air is fed to them from electrically powered blowers. As long as this airflow is maintained, stack gas does not penetrate into the sight tubes. Typically the sight tubes end in flanges. The two sides of the monitoring instrument are bolted to these flanges. Each side of the instrument has a closure window. These windows stay clean because stack gas does not reach them.
It is good practice to interpose a ‘fail-safe shutter’ between the flange and the instrument at both sides. The purpose of this shutter is to close off the end of the sight tube if the purge air supply should fail for any reason.
Various designs of shutter are commercially available. A good design achieves the following:                Positive closure when the purge air flow stops.        Positive closure when the electrical power supply fails (this being a primary reason for loss of purge air—the blowers stop).        A good seal against the stack gas when closed        Seal protection of the fail-safe shutter mechanism as well as the monitoring instrument        Automatic re-opening when the purge air flow resumes        High reliability        Small size—in particular along the direction of the sight path        
It is surprisingly difficult to achieve all the above in practice.
Some known designs are based on the purge flow itself holding a flap open against a spring. This arrangement may be elegant but does not always perform well in practice. Purge air is typically sourced from blowers rather than a compressed air supply. Available pressure is thus very limited, allowing only a weak spring force at the flap. This can lead to unreliable closure, poor sealing, and a tendency to ‘stick’ either open, closed or half open.
Other known designs use a solenoid to hold a gate open against a drop-weight. A flow sensor detects a loss of purge air, de-energising the solenoid and allowing closure. The same happens if the electrical power fails. Such designs can work well but require manual re-setting. This involves someone climbing the stack to lift the gate against the drop weight.
Similar known designs use a solenoid to hold a flap open against a spring. They suffer the same problems.
Other known designs use a motorised ball-valve in conjunction with a sensor which detects purge airflow. These can be effective but tend to be large and very expensive. In part this is due to the size of ball valve required to pass the required optical beam, in part to the need for a back-up power supply to allow the valve to be driven closed if the main electrical supply fails.
Similar designs use a motorised gate valve. They suffer the same problems.
Summary of a First Aspect of the Invention
                According to a first aspect of the present invention there is provided, a fail-safe shutter for a “cross-stack” monitor, comprising        (i) a body member having an aperture through which, in use, an associated monitoring instrument is adapted to sight;        (ii) a gate movable between a retracted, open position in which the aperture is exposed, and a closed position in which the aperture is closed-off by a portion of the gate;        (iii) means to bias the gate towards its closed position; characterised in that the shutter further comprises        (iv) an electric motor, a gearbox and a magnetic clutch, the clutch, when energised, connecting the gate to the motor) via the gearbox, and the motor, when energised, serving to drive the gate via the gearbox and the energised clutch to an open position, with attainment of the latter de-energising the motor only, and with the bias means being insufficient to overcome the mechanical advantage of the gearbox, whereby the gate is retained in its open position until such time as the clutch is de-energised either by a power failure, or by purge gas sensing means detecting absence of purge gas, or by other means; and        (v) control circuitry to maintain, and/or interrupt and/or return electrical supply to the clutch, and/or to the electric motor.Summary of a Second Aspect of the Invention        
According to a second aspect of the invention, there is provided a cross-stack monitor incorporating a fail-safe shutter in accordance with the first aspect.